This invention relates to nonwoven fabrics. More particularly, the present invention relates to multilayer nonwoven fabric composites of thermoplstic resin fibers, wherein the fibers making up at least one layer are at least in part flat-crimped bicomponent fibers, and to a method for manufacturing such composites. The nonwoven fabric composites of the present invention are configured in such a way as to be especially useful as coverstock in absorbent articles such as disposable diapers and sanitary napkins.
Disposable diapers, sanitary napkins, and the like are generally composed of an impermeable outer covering, an absorbent layer, and an inner layer that--ideally--permits liquid to flow through it rapidly into the absorbent layer ("rapid strike through") but does not permit or at least does not facilitate re-transmission of liquid from the absorbent layer to the "baby" or "wearer" side of said inner layer ("resists rewet"). Said inner layer is referred to as coverstock, topsheet, or, in diaper applications, diaper liner. In addition to liquid transport properties, the coverstock must have sufficient strength to allow for converting it--that is, incorporating into the final product--on a diaper or other machine and for resistance to failure during vigorous movement by the user. On the other hand, while strength is essential, the coverstock should present a soft comfortable feel against the user's skin. Currently these somewhat conflicting requirements--for softness coupled with strength--have been met only imperfectly, for the most part by coverstock made from thin low basis weight (in the neighborhood of 20 grams per square yard) carded or spunbonded nonwoven fabrics.
It has been recognized that many aspects of coverstock performance could be substantially improved if the thickness, or caliper, of the coverstock fabric were increased. The subjective feel--softness and dryness--of diaper liner has become more important with the increased use of diapers by incontinent adults. Surface dryness generally can be improved by increasing the separation between the wearer's skin and the absorbent core of the diaper. Since this separation must be maintained during use of the diaper it is essential that the thick diaper liner maintain its caliper under some degree of compression loading. Thickness can be increased by increasing the basis weight of the coverstock and/or by decreasing the density thereof (that is, by making the coverstock more "lofty"). Increased thickness through loft should offer improved softness as well as improved surface dryness.
Many approaches have been suggested for producing thick diaper liner. For example, U.S. Pat. No. 4,041,951 teaches embossing nonwoven topsheet to increase its bulk, and U.S. Pat. No. 4,391,869 discloses limiting the amount of aqueous binder applied in the suction bonding of airlaid nonwoven fabric. More recently, the use of thru-air bonded bicomponent fiber structures have been investigated. One use of the thru-air technique is alluded to in an article entitled "Multi-layer Nonwovens for Coverstock, Medical, and other End Uses" by J. Pirkkanen in the November 1987 issue of "Nonwovens World". The reference discloses a multilayer nonwoven fabric having a basis weight of about 30 grams per square meter. U.S. Pat. No. 4,548,856 and U.K. patent application GB No. 2,127,865A disclose thru-air bonding procedures that involve the use of multibelt systems to form patterned nonwoven fabrics.
U.S. Pat. No. 4,652,484 assigned to Kao teaches that improved diaper liner will result from layered structure wherein the first layer is predominently comprised of 1-3 denier straight bicomponent fibers and the second layer is predominently comprised of sterically buckled (three-dimensional crimp) 1.5 to 6 denier bicomponent fibers. It is believed that what the patent refers to as "straight" fibers are actually flat-crimped fibers. The method used to bond this structure is not disclosed in detail. The diaper liner used in commercial MERRIES diapers, which are produced by Kao have a basis weight of approximately 30 grams per square yard. The examples of diaper liners according to the invention disclosed in U.S. Pat. No. 4,652,484 have basis weights of 35 grams per square yard. Thus the Kao structure suggests the need for a 50% increase in basis weight over conventional (20 grams per square yard) diaper liner. Also, the Kao patent teaches that optimum thickness and softness is achieved only if the structure is complicated by the use of sterically-buckled bicomponent fibers.
I have now unexpectantly found that diaper liner having properties of thickness, softness, and strength comparable to the Kao products can be manufactured using flat-crimped (rather than sterically-buckled) bicomponent fibers. Moreover, I can achieve such results at substantially reduced basis weights compared to the basis weights of comparable webs described by the Kao patent.